Affibodies are relatively small proteins (compared to antibodies or antibody fragments) that can be radiolabeled and used to image specific targets in vivo. Drs. Jacak Capala (NCI) and Dale Kiesewetter (NIBIB) have synthesized F-18 labeled affibodies (F-18FBEM-Z-HER2:342) to image the HER2 receptor that is found in several types of cancer, including breast and lung. HER2 positivity is associated with resistance to certain types of treatment, so the ability to image the HER2 receptor in cancer patients could be useful in the selection of appropriate cancer treatment. In vitro and rodent studies with F-18FBEM-Z-HER2:342 have shown high affinity to the HER2 receptor, good clearance from blood, and high accumulation in HER2 positive tumors, suggesting it might be useful as a PET radiopharmaceutical in humans. Based on these studies in rodents, preclinical studies with F-18FBEM-Z-HER2:342 were performed in nonhuman primates to determine its biodistribution, clearance from blood and pathways of excretion. Biodistribution data were analyzed. There was rapid clearance of the tracer from blood, as was anticipated with an affibody. Tracer excretion was through the urinary tract, with no hepatobiliary excretion observed. There was retention of tracer in kidney and liver. Radiation dose estimates extrapolated to humans were calculated with OLINDA software. Preliminary results indicated organs with the greatest radiation exposure were the liver (0.70 rad/mCi), kidney (0.65 rad/mCi), and urinary bladder (0.35 rad/mCi) These radiation dose estimates should permit human doses to be administered that are suitable for clinical PET imaging.